There are two places where this can happen:

Parser.java: method parse(InputSource input) - line 492:

        catch (SAXException e) {
            reportError(ERROR, new ErrorMsg(e.getMessage()));
        }

ErrorMsg(String) expects a localization bundle key - a _code - but is passed an exception message.

TransformerFactoryImpl.java: method newTemplates(Source source) - lines 789-792:

            catch (Exception e) {
                ErrorMsg err = new ErrorMsg(
                                     new ErrorMsg(ErrorMsg.RUNTIME_ERROR_KEY)
                                     + e.getMessage());
                throw new TransformerConfigurationException(err.toString());
            }

Here again the first ErrorMsg(String) expects a localization bundle key - a _code - but is passed an exception message.

This affects version 8 - but the fix can probably be backported to version 7...